WebSocket's MD methods will fail when input is backed by non-zero offset array

The message digest methods of `WebSocketUtil` will fail when the specified input buffer is backed by array that have non-zero offset. Attached test can produce the following failure.

```
-------------------------------------------------------------------------------
Test set: org.jboss.netty.handler.codec.http.websocketx.WebSocketUtilTest
-------------------------------------------------------------------------------
Tests run: 2, Failures: 2, Errors: 0, Skipped: 0, Time elapsed: 0.082 sec <<< FAILURE!
testSha1(org.jboss.netty.handler.codec.http.websocketx.WebSocketUtilTest)  Time elapsed: 0.018 sec  <<< FAILURE!
java.lang.AssertionError: expected: org.jboss.netty.buffer.BigEndianHeapChannelBuffer<BigEndianHeapChannelBuffer(ridx=0, widx=20, cap=20)>
but was: org.jboss.netty.buffer.BigEndianHeapChannelBuffer<BigEndianHeapChannelBuffer(ridx=0, widx=20, cap=20)>
	at org.junit.Assert.fail(Assert.java:93)
	at org.junit.Assert.failNotEquals(Assert.java:647)
	at org.junit.Assert.assertEquals(Assert.java:128)
	at org.junit.Assert.assertEquals(Assert.java:147)
	at org.jboss.netty.handler.codec.http.websocketx.WebSocketUtilTest.testSha1(WebSocketUtilTest.java:58)
	...
testMd5(org.jboss.netty.handler.codec.http.websocketx.WebSocketUtilTest)  Time elapsed: 0 sec  <<< FAILURE!
java.lang.AssertionError: expected: org.jboss.netty.buffer.BigEndianHeapChannelBuffer<BigEndianHeapChannelBuffer(ridx=0, widx=16, cap=16)>
but was: org.jboss.netty.buffer.BigEndianHeapChannelBuffer<BigEndianHeapChannelBuffer(ridx=0, widx=16, cap=16)>
	at org.junit.Assert.fail(Assert.java:93)
	at org.junit.Assert.failNotEquals(Assert.java:647)
	at org.junit.Assert.assertEquals(Assert.java:128)
	at org.junit.Assert.assertEquals(Assert.java:147)
	at org.jboss.netty.handler.codec.http.websocketx.WebSocketUtilTest.testMd5(WebSocketUtilTest.java:43)
	...
```